Comparative Study of Physical Models for Particle Sedimentation Using SIMMER Code

In the context of improved safety requirements for Generation IV sodium-cooled fast reactors (SFR), an innovative severe accident mitigation scenario is being investigated. In the French frame of SFR research, the mitigation strategy consists of transfer tubes and a core catcher. The transfer tubes are dedicated to discharge molten fissile materials from the core center region and to guide them toward the core catcher where long-term cooling and subcritical state may be assured. The physical phenomena occurring during the discharge process are introduced in this paper. The current demonstration of the mitigation strategy uses best-estimate calculations with the reference computer code SIMMER. Previous analyses showed that the material discharge through the transfer tubes might be efficient; however, uncertainties of SIMMER approach are identified on the molten material mobility during the relocation process. It is related to a blockage formation due to particulate solid debris accumulation inside the transfer tube, in case of low energy accumulation in the degraded fuel, is believed to originate from the solid particle treatment in the code. As the performance of mitigation strategy strongly depends on the mobility of the relocating mixture, the most predictive behavior of particle flows is of great importance to SFR safety. Therefore, the SIMMER modeling of such flows is analyzed in this work. The first verification and validation test cases regarding the gravitational settling of particle clouds at varying volume fractions are presented. Recommendations for reactor calculations and first orientations for future research and development are highlighted. (authors)